Method of replacing subassemblies in nuclear reactors



March ,1 J. G. YEYICK ETAL 3,503,849

METHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS Original FiledJan. 7. 1966 ll Sheets-Sheet 1 TIL March 31, 1970 J. G. YEVlCK ET ALMETHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS ll Sheets-Sheet 2Original Filed Jan. '7. 1966 INVENTORS JOHN G. yfV/CK fol-(n90 r: (S /LMarch 1970 I J. G. YEVlCK ETAL 3,503,849

METHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS Original FiledJan. 7, 1966 11 Sheets-Sheet 3 I VENTORS Ti q S JOHN yf'y/C K Mar 1970J. G. YEVICK ETAL 3,503,849

METHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS l1 Sheets-Sheet 4Original Filed Jan. 7, 196$ TORS JOHN G. YEV/(K INVEN [Oh A90 F 59/1. c

March 31,- 1970 J. G. YEVICK T L 3,503,849

' METHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS Original FiledJan. 7,1965 11 Sheets-Sheet 5 ZIO"? 214 Elllf ZIZ--- I Q r n I Z )IZZINVENTORS \Zb 24 JOHN G. YFV/CK March 31, 1970 J. G. YEVICK ETAL3,503,849

METHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS Original FiledJan. 7, 1966 11 Sheets-Sheet 6 INVENTORS JUHN 6. YEV/C/f March 31, 1970J. G. YEVICK ET A 3,503,849

METHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS ll Sheets-Sheet 7Original Filed Jan. 7, 1966 INVENTORS JOHN 6. Y5 v/(K 0 4 4 0 F G /LAMETHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS Original FiledJan. '7, 1966 March 31, 1970 I J, v c ET AL 11 Sheets-Sheet 8 INVENTORSJOHN 6. YEV/C/T Zea/4 o F 51 /44 March 31, 1970 J vlc ETAL 3,503,849

METHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS Original FiledJan. 7. 1966 ll Sheets-Sheet 9 Ti. E 2.

INVENTORS 48 JOHN G. YEV/CK March 31, 1970 J. G. YEVICK ETAL 3,503,849

METHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS Original FiledJan. 7. 1966 ll Sheets-Sheet 1O INVENTORS JOHN G. YEV/CK flaw/$90 f 511/14.

J. 5. YEVICK ETAL 3,503,849

March 31, 1970 METHOD OF REPLACING SUBASSEMBLIES IN NUCLEAR REACTORS 11Sheets-Sheet 11 Original Filed Jan. '7, 1966 r mwm w ymw wcwmfi 0/} 0 Www NW 64560164 Fbsmo/y @IY G Ema/raw;-

United States Patent Int. Cl. G21c 19/20 US. (:1. 176-30 4 ClaimsABSTRACT OF THE DISCLOSURE A method of replacing a subassembly in anuclear reactor including: positioning a subassembly handling mechanismon top of the main plug and aligning it with a preselected liner tube,removing a small top plug from engagement with the top of the main plugwith a gripper cable mechanism and positioning same within thesubassembly handling mechanism, and lowering a new subassembly into theliner tube. The next steps are: retaining the new subassembly inside theliner tube in a decay region, removing a hot subassembly from the coreof the reactor and placing it in the decay region adjacent the newsubassembly, and thence placing the new subassembly in the reactor core,and removing a decayed subassembly which has been retained in the decayregion since the last refueling operation, then returning the small plugfrom the'subassembly handling mechanism to its engaged position on topof the main plug, and removing the subassembly handling mechanism.

This application is a division of our copending application, Ser. No.519,235, filed Jan. 7, 1966.

This invention pertains to a nuclear reactor structure and to methods ofoperating same. Nuclear reactors entbodying the concept of thisinvention are particularly adapted, among other possible uses, for usewith a fast neutron reactor system.

The invention pertains more particularly to improvements in thestructure and arrangement of the components in a fast nuclear reactorstructure and to new and improved methods of operating and refueling thereactor.

The invention has, as one of its objectives, a simplification ofapparatus and methods heretofore deemed necessary for operating andrefueling reactors of the character aforesaid, whereby apparatuscomponents and manipulative operations are eliminated as compared topractices of the prior art.

These and other advantages of the apparatus and methods of the inventionas compared to nuclear reactor systems heretofore utilized will becomeapparent as the description proceeds.

A feature of the present invention resides in the provision of a new andimproved nuclear reactor which has improved accessibility to the fuelelements, control elements and safety rods disposed in the reactor coreand in the reactor blankets, which simplifies the fuel and controlsubassembly handling and replacement procedures, and which does notrequire side or bottom penetration of the reactor vessel for maintenanceoperations or control equipment.

Another feature of the invention resides in the provision of a nuclearreactor which reduces or eliminates fuel element bowing, which permitsindividual instrumentation of the subassemblies, which does not requireremoval of all control and drive mechanisms during refueling operations,and which permits monitoring and control of coolant fiow and coolanttemperatures of each subassembly.

Still another feature of the invention is the provision of a nuclearreactor which permits storage of the subassemblies in liner tubes abovethe reactor core, which provides a simple means for storingsubassemblies for decay within the reactor vessel itself, which permitsclose spacing of the fuel subassemblies in the reactor core to save fueland permits sufficient spacing of the fuel subassemblies towards theupper end of the reactor for accessibility purposes, and which minimizesthe number of steps required to refuel the reactor.

Moreover, another feature of the present invention resides in theprovision of a new and improved nuclear reactor which does not require arotary top plug, which provides simple handling mechanisms, and withwhich it is possible to arrange hexagonal subassemblies in a stablearray so that any of the subassemblies can be removed from one of theliner tubes through a small top plug.

Another feature of this invention is the provision of a new and improvednuclear reactor with which the use of liquid control means having failsafe features may be employed, which provides interruptions in thestreaming of neutrons from the core by transition sections, whichreduces the volume of sodium required within the reactor vessel, andwhich limits the number of steps and simplifies the mechanism requiredfor refueling of the reactor.

In order to achieve the aforementioned features and objects, thisinvention contemplates the provision of a nuclear reactor having areactor core and blanket portions, a plurality of liner tubes, eachcontaining a plurality of subassemblies which are disposed in anupwardly diverging conical array. Each of the sleeves has asubstantially constant cross-sectional area throughout the core andblanket portions of the reactor.

Further, means are provided for removing said subassemblies from saidliner tubes.

The invention in one form thereof contemplates the provision of anuclear reactor having a plurality of sleeve members, each sleeve memberhaving eighteen sides, twelve of which are equal and longer than theother six equal sides. Seven hexagonal subassemblies are positionedwithin each of the sleeve members and the sleeve members are disposed ina substantially upwardly diverging conical array.

The invention in another form thereof comprises a nuclear reactor havinga first region which contains the reactor core and blanket portions anda second super adjacent region which contains a core decay region. Aplurality of liner tubes are provided, each of which contains aplurality of subassemblies. Each of the tubes has a substantiallyconstant cross-sectional area throughout the portion thereof disposedwithin the first region and a larger second substantially constantcross-sectional area throughout the portion thereof disposed within thesecond region. Means are provided for moving selected fuel subassembliesfrom the first region to the second region and means are provided forretaining said selected subassemblies within the second region.

The invention in still another form thereof comprises a nuclear reactorcomprising a first region which contains a reactor core and blanketportions. A plurality of closely spaced liner tubes are disposed in saidregion and each of said tubes contains a plurality of subassemblies.Fluid material is contained in the space between the liner tubes, andthe level of this material in the reactor core is controlled in order tocontrol the energy level of the nuclear reactor. The level is controlledby means of supplying a gas under pressure which acts on the surface ofthe fluid material to force the fluid material from the reactor core toa location remote therefrom. A large quantity of fluid material disposedin the reactor core tends to slow down the nuclear reaction; and a smallquantity thereof allows the reaction to speed up.

The invention in another form thereof comprises a method of replacing asubassembly in a nuclear reactor comprising loading a new subassemblyinto the subassembly handling mechanism and positioning the mechanism ontop of the main plug of the nuclear reactor aligned with the opening inthe corresponding liner tube, the feet of the base being guided inhemispherical guide holes disposed on top of the main plug. The methodfurther comprises evacuation and purging the space between the top ofthe main plug and the valve of the subassembly handling mechanism,opening the valve of the subassembly handling mechanism, connecting acable to the small top plug of the nuclear reactor by a grippermechanism and removing the small top plug to a bell housing disposedwithin the subassembly handling mechanism, and moving the bell housingby an actuator system to a remote position.

The next steps comprise lowering a new subassembly into the liner tubeand hooking same on the inside of the round portion of the liner tube bymeans of an electrically operated self-seeking gripper head of theparallelogram type. The subsequent steps include removing a hotsubassembly from the core of the nuclear reactor and placing same in astorage position adjacent the new subassembly, then placing the newsubassembly in the reactor core, and moving a decayed subassembly, whichhas been retained in the decay region for a substantial period of time,to a rack disposed within the outer housing of the subassembly handlingmechanism, returning the bell housing to its position adjacent the mainplug, lowering the small plug by the cable mechanism to its closedposition, closing the valve of the handling mechanism, sweeping thespace between the valve and top of the small plug with an inert gas, andfinally moving the subassembly handling mechanism to a remote location.

There has thus been outlined rather broadly the more important featuresof the invention in order that the detailed description thereof thatfollows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,addi tional features of the invention that will be described hereinafterand which will form the subject of the claims appended hereto. Thoseskilled in the art will appreciate that the conception on which thisdisclosure is based may readily be utilized as a basis for the designingof other structures for carrying out the several purposes of theinvention. It is important, therefore, that the claims be regarded asincluding such equivalent constructions as do not depart from the spiritand scope of the invention. Specific embodiments of the invention havebeen chosen for purposes of illustration and description, and are shownin the accompanying drawings, forming a part of the specification,wherein:

FIG. 1 is a vertical sectional view of a nuclear reactor constructed inaccordance with the concepts of this invention;

FIG. 2 is an enlarged horizontal sectional view taken along the line 22of FIG. 1 and showing portions of the liner tubes and subassembliesdisposed in the reactor core;

FIG. 3 is an enlarged fragmentary horizontal crosssectional view of amedial portion of FIG. 2;

FIG. 4 is an enlarged horizontal sectional view taken on the line 4--4of FIG. 1 and showing portions of the liner tubes and subassembliesdisposed in the upper axial blanket region;

FIG. 5 is an enlarged horizontal sectional view taken on the line 55 ofFIG. 1;

FIG. 6 is an enlarged horizontal sectional view taken along the line 66of FIG. 1 and showing portions of the liner tubes and subassembliesdisposed in the core decay region of the nuclear reactor;

FIG. 7 is an enlarged horizontal sectional view taken along the line 7-7of FIG. 1;

FIG. 8 is an enlarged cross-sectional view taken along the line 8-8 ofFIG. 1 and showing a plurality of subassemblies disposed within a linertube in the core decay region;

FIG. 9 is an enlarged detailed horizontal sectional view showing theconstruction of the polygon portion of the liner tube and the coolantoutlet openings;

FIG. 10 is an enlarged vertical sectional view showing the details ofconstruction of the lower end of the liner tube and the support meanstherefor;

FIG. 11 is a detailed vertical sectional view showing the constructionof the lower portion of a subassembly and hold-down means therefor;

FIG. 12 is a side elevation showing somewhat schematically the liquidlevel control system for the nuclear reactor;

FIG. 13 is an enlarged horizontal sectional view taken substantiallyalong the line 1313 of FIG. 12 and showing the liquid reservoir as wellas the gas supply and vent line;

FIG. 14 is an enlarged horizontal sectional view taken substantiallyalong the line 1414 of FIG. 12 and showing the space between liner tubesfor the control liquid, and also showing the connecting tube leading tothe liquid reservoir.

FIG. 15 is a plan view of the liner tube showing the gas supply and venttube;

FIG. 16 is a horizontal sectional view of a liner tube showing analternate embodiment employing a single gas supply connection;

FIG. 17 is an enlarged horizontal sectional view of the polygon portionof the liner tube showing still another embodiment of the fluid levelcontrol system;

FIG. 18 is a side elevation, partially in section, of a subassembly inits storage or decay position within its liner tube;

FIG. 19 is an enlarged view showing details of the supporting structurefor holding the subassembly in position for storage or decay within theliner tube;

FIG. 20 is a side elevation showing the subassembly and small plughandling mechanism;

FIG. 21 is an enlarged detailed view showing the parallel axis gripperfor the subassembly handling mechanism;

FIG. 22 is a side elevation, partially in section, of the top of a linertube, a small plug, and means for locking the small plug in closedposition;

FIG. 23 is a side elevation of a liner handling mechanism;

FIG. 24 is a common base for the subassembly and the liner handlingmechanisms;

FIG. 25 is a plan view of the main plug showing the hemispherical holesfor locating the common base of the subassembly and liner handlingmechanisms;

FIG. 26 is a plan view showing schematically a system' for replacing asubassembly; and

FIG. 27 is a plan view showing schematically a system for replacing acontrol rod and a fuel subassembly or two fuel subassemblies.

In the embodiment of the invention illustrated in FIG. 1, the nuclearreactor includes a reactor vessel 10, which is preferably fabricatedfrom stainless steel plate with stainless steel rings at transitionsections of the vessel. While a cylindrical vessel is illustrated inFIG. 1, the sidewalls of the vessel could be tapered downwardly andinwardly in order to reduce the required volume of liquid coolant whichmay be liquid sodium, for example. A thermal baffle 12 is disposedinwardly of the reactor vessel 10, and a secondary containment tank 14is disposed outwardly thereof. Insulation material 16 forms the outerlayer of the reactor wall. Coolant inlets 18 are provided at the foot ofthe reactor through which the coolant, which may be sodium for example,passes to the inlet plenum 20. Still referring to FIG. 1, bellows 22 areprovided in the inlets 18 for expansion purposes. A torispherical head24 is provided as the closure means at the bottom of the reactor, andadjacent thereto is a melt-down pan 26 which can be provided with anencapsulated poison control material on the underside to prevent acritical mass formation in the melt-down pan.

The core 30 is centrally located in the reactor and is illustrated bythe dotted lines in FIG. 1. FIG. 2 is a horizontal sectional view takenalong the line 22 of FIG. 1 and shows additional details of the core. Asseen in FIG. 1, the core 30 is surrounded by a radial blanket designatedgenerally at 32, a lower axial blanket 34 and an upper axial blanket 36.An isolation region 38 is located above the upper axial blanket 36 forstoring or decaying the subassemblies. The reactor is provided with anoutlet or exit plenum 40 and coolant outlets 42. The reactor is furtherprovided with superstructure indicated generally at 46 for insertion andwithdrawal of the fuel and control elements. The superstructure includesa large main plug 48 which serves as a closure means for the top of thereactor.

The reactor vessel is filled with a plurality of elongated verticallyextending liner tubes designated generally at 50 (FIG. 1) in which thesubassemblies are disposed. The liner tubes are positioned in asubstantially upwardly diverging conical array. Each liner tube isconstituted with a lower polygon tube portion 52 having eighteen sides,twelve of which 54 are equal and longer than the other six equal sides56, to permit closer spacing of the liner tubes, as best seen in FIG. 4.The polygon-shaped portion 52 of the liner 50 is joined by a transitionsection ring 56 (FIG. 1) to a second section 58 of the liner tube 50. Asbest seen in FIGS. 5, 6 and 8, the second section 58 of the liner tube50 is of circular figuration. As best seen in FIG. 7, which is asectional view taken along the line 7-7 of FIG. 1, the liner tube 50further comprises a third upper section 60 which is of roundconfiguration, but of a diameter greater than the section 58. It isnoted that all core and radial blanket liners are identical.

It will be appreciated that in a fast reactor the power is concentratedin a relatively small volume in order to save critical material, andhence the structure is of particular importance. Considerable effectsare caused by expansion, bowing or buckling of the structure. Accordingto the present invention the liner tubes are disposed in a substantiallyconical array. That is, the liner tubes are closely spaced at the bottomof the reactor in the core region and they are spaced further apart atthe top of the reactor for purposes of removing the fuel and controlsubassemblies. Preferably there is a conical angle of the order of about0.8 between one liner tube with respect to the next adjacent liner tube.The subassemblies disposed within each liner tube are substantiallyparallel one with respect to the other, and therefore, the actualstreaming loss is decreased.

The liner tubes 50 pass through the plug 48 and are free to expand asmall amount within the plug, the top ends of the liner tubes beingradially guidedby the top of the plug. As best seen in FIG. 1, a secondradial support grid 62 is located at the transition section 56 which maybe, for example, about three feet above the top of the upper axialblanket 36. The lower ends of the liner tubes 50 are free to moveradially with bowing of the core. This radial freedom is provided bymeans of bellows 64, disposed at the bottom of each liner tube where itengages the support plate 66, the liner tubes being axially fixed at thebottom. This arrangement produces a negative bowing coeflicient and anegative liner tube axial expansion coefficient. Referring to FIG. 10,the liners 50 have a small spherical shaped bottom tube or ring 68 whichfits into openings 70 in the pressure plate 66. Each liner tube isprovided with a flow guide tubular member 72 for guiding the incomingliquid coolant or molten sodium, as shown by the arrows in FIG. 10. Eachof the liner tubes 50 has an internal support plate 74 for supportingthe bottom of the subassemblies, designated generally at 98. It is notedthat the term subassembly as used herein broadly encompasses controlrods and fuel subassemblies, both of which have external contours suchas to make them interchangeable. The fuel subassembly may include manydifferent forms of reactor fuels, for example it may include a solidfuel pin or a wrapper can filled with fuel pellets.

According to one embodiment of the present invention, hexagonalsubassemblies 76 are arranged in a stable array in groups of seveninside each liner tube 50 as best seen in FIGS. 3, 4, 5 so that any oneof the subassemblics can be removed from its respective liner tubethrough a small top plug. The core 30 (FIG. 1) has a central liner orliner tube surrounded by six similar liners which are surrounded bytwelve additional liners, thereby providing 133 core subassemblies. Theradial blanket region adds another eighteen liner tubes in the first rowand twenty-four in the second row thus, the total number of liner tubesfor the core and radial blanket is sixty-one, and hence space isprovided for a total of 427 subassemblies. This requires sixty-one toppenetrations of the nuclear reactor for control and refuelingprocedures.

Referring to FIG. 25, the main plug 48 is provided with sixty-one roundequilaterallyspaced top penetrations 82 for providing access to theliners containing the subassemblies. As seen in FIG. 22, a plug tube orsleeve 84 surrounds each liner 50 and is welded to the top of the plug48 as at 86, as well as to the bottom of the plug, thereby effectivelysealing the plug. Reverting to FIG. 1, the interior of the plug 48includes carbon 88, or other shielding material, as well as insulationmaterial and steel in the form of powder or granules which are compactedby careful procedure to fill all void spaces.

Actually, the plug 48 is a section of a hollow sphere, the under sideand the outer or top side each having a radius of curvature. The holes82 (FIG. 25), for the liner tubes are machined so that the center lineof the holes passes through the center of the theoretical sphere givinga generally conical effect. A fuel handling mechanism is provided whichfits over the holes 82 and forms a tight joint with the plug 48 duringthe refueling operation. The tube liners 50 which pass through the plug48 are free to expand a small amount within lower end of the tubularplug sleeves 84. The top ends of the liner tubes 50 are radially guidedby the sleeve 84 as seen in FIG. 22. An O-ring 85 serves to seal tube 50with respect to sleeve 84. The tube liners are removable through theirrespective hole 82 in the plug 48 for purposes of replacement in theevent of structural damage to the tubes during the life of the reactor.

Still referring to FIG. 22, small top plugs 92 are provided for closingthe top of each liner tube 50 and are held in position by self-lockingclamps 94. A pair of 0- rings 96 form an effective seal between the topplug 92 and the plug sleeve 84. Instrumentation and monitoring of eachindividual subassembly may be handled through the small top plugs 92, ifdesired.

The coolant fluid, Which may be sodium for example, enters the reactorat the inlets 1-8 (FIG. 1) and passes to the inlet plenum 20. Thence,the flow of sodium enters the flow guide 72 (FIG. 10) which passesthrough the pressure plate 66 and enters the liner tube at the bottomthereof. After which it divides into seven parallel paths one for eachsubassembly and passes upwardly therethrough. After passing out throughthe top of the subassemblies, the coolant is radially discharged fromthe liner tubes 50 through holes or slots 78 (FIG. 9) towards the top ofthe sides of the polygon-shaped portion 52 of the liner tubes into thelarge sodium pool (FIG. 1) surrounding the liner tubes. Thence, thesodium passes through the outlet plenum 40 to the outlets 42 and on tothe other components of the power system. The sodium above the supportor pressure plate 66, can be drained from the reactor vessel by removingone or more of the liner tubes 50. The support plate 66-, all thermalbaffling and the melt-down pan 26 can be removed from the top of thereactor after the main plug 48 and all other inner members have beenremoved.

Referring again to FIG. 10, each subassembly 76 is provided with a lowersubassembly nozzle designated generally at 98 through which passes theaforementioned coolant fluid. Due to the buoyancy of the coolant fluidin the pool 80, the pressure of the coolant in the inlet plenum 20* aswell as the flow characteristics of the coolant there is an upward forcecomponent tending to lift the subassemblies. Therefore, a hold-downmechanism is necessary. However, such mechanism should permit axialexpansion of the core and blanket subassemblies due to thermalexpansion. As best seen in FIG. 11, the hold-down mechanism comprises asubassembly orientation cam 104, a cushion spring 106, a spring retainer108, and a spring guide 110. The nozzle 98 is inserted into a fixedportion 112, which is part of support plate 74. The hold-down mechanismfurther comprises leaf springs 114, a delatching stop 116, a cam surface118, a delatching holding ring 120, a latching thimble having a circularband 122, and a leaf spring latch 124. In order to lock the nozzle inits operating position within the liner tube it is pushed downwardlythrough the delatching holding ring 120 and the latching thimble 122,thereby depressing the six leaf springs 114 until they are past thecircular band of the latching thimble 122, where they snap out andengage the thimble as at 126. When it is desired to remove thesubassembly from its position in the liner tube 50, the nozzle 98 isinitially pushed downwardly until the delatching holding ring 120engages the cam surface 118 of the leaf spring 114, thereby disengagingthe latches from the latching thimble 122. As the subassembly and nozzleare raised, the delatching stop 116 disengages the holding ring 120 andthus allows the nozzle to be removed. It is noted that the compressionspring 106 acts between the upper portion of the support member 74 andthe spring retainer 108 to provide axial tension in an upwardlydirection on the nozzle 98, thereby resiliently retaining thesubassembly 76 in position.

Alternately, the small top plugs 92 (FIG. 22) may be equipped withsubassembly hold-down extensions, each of which may engage all thesubassemblies in each liner tube to prevent them from moving upwardduring reactor operation.

Reactor control can be provided by replacing the central coresubassemblies in selected liners by moving control rods. That is,control rods can be placed in selected liners at the center between fuelsubassemblies. The operation of these control rods can be effected by amechanism on the top of the small liner plugs 92- in a normal manner,the control rods being interchangeable with the fuel subassemblies.

In another embodiment of this invention control of the reactor iseffected by means of regulating the level of liquid lithium disposed inthe space between the liners 50. As best seen somewhat schematically inFIG. 12, the core portion 52 of the liner tube 50 is surrounded by abath 126 of lithium which is connected through a tube 128 to an upperreservoir 130 of lithium. The bath 126 is formed by means of analternate liner tube 132 (FIG. 14) and the tube 128- actually passestherethrough as shown. The reservoir 130 (FIG. 12) is formed by a wallmember 134 and a gas supply and vent pipe 136 passes therethrough. Thepipe 136 extends upwardly adjacent the side wall of the polygon portion52 of the liner tube as seen by the dotted line in FIG. 15. As the pipeproceeds upwardly through the round portion of the liner tube it isangled outwardly as shown by the dotted lines in FIG. and at the top ofthe liner tube 60 the pipe 136 exits from the small top plug 92 as shownin FIG. 22. Preferably an inert gas is employed such as argon, forexample. In operation the tube 136 contains gas under pressure whichflows inwardly to displace the lithium in the space 126 (FIG. 12)surrounding the polygon liners 52. The displaced lithium fiows upwardlythrough the pipe 128 to the reservoir 130, and hence less lithium is inthe area immediately surrounding the reactor core. It will beappreciated that lithium is a neutron absorber and hence the presencethereof slows down a nuclear chain reaction. Thus, the presence oflithium adjacent the reactor core slows down the reaction and theabsence thereof allows the reaction to speed-up. When additional lithiumis desired in the core region, the pressure in the gas supply line 136(FIG. 12) is reduced and lithium flows downwardly from the reservoirthrough the tube 128 to the bath 126 surrounding the core portion 52 ofthe liner 50. It is noted that this embodiment requires six gas supplylines for each liner assembly. Another embodiment is shown in FIG. 16which requires a single gas connection 140 for all of the subassembliesin a liner. In this embodiment the pipe 136 of FIG. 12 is dispensed withand the upper round sections 58 and 60 of the liner tubes 50 areprovided with thin concentric walls 142 and 144 between which the gasflows downwardly and into contact with the upper surface of the bath 126(FIG. 12). Concentric tube spacers 146 (FIG. 16) are supplied to keepthe two tubes spaced one with respect to the other. The operation ofthis embodiment is similar to that described above in connection withFIGS. 13-15 inclusive.

Referring to FIG. 17 there is shown another form of the inventionwherein the lithium space 150 corresponding to the lithium space 126(FIG. 12) is formed by a conical polygon wrapper 148 concentricallydisposed about the polygon portion 52 of the liner tube, said wrapperbeing connected to the polygon portion 52 by spacers 154, thisembodiment operates in a similar manner to that described in connectionwith FIGS. 13-15 inclusive and pipes 152 correspond to pipes 128. Due tothe closer spacing of the liner tubes towards their lower ends, pipes152 may be flattened at their lower ends in order to fit between theliner tube Wall and the conical polygon wrapper.

It will be appreciated that in the aforementioned lithium controlsystems there is a safety feature wherein the device is what is known asfail safe. That is, in the event of mechanical failure resulting in lossof gas pres sure, the liquid lithium will automatically drain to thereactor core and slow down the nuclear reaction. Upon restoration of thegas pressure the reactor will automatically return to normal operation.

It will be appreciated that the subassemblies 50 have to be replacedperiodically, and therefore, means are provided for their removal andreplacement. Also, during the life of the reactor, the liner tubes mayrequire replacement, and hence provision for their replacement isnecessary. Referring to FIG. 24, there is illustrated a common supportbase designated generally at 156- which is used for both the fuelsubassembly handling mechainsm and the liner handling mechanism. Thisbase comprises a crane lifting bar 158, a trunnion support 160 for fuelsubassembly handling and liner handling mechanisms and a locking pinsolenoid 162. A spherical pivot 164 is provided having an angleadjusting actuator 166 and a raising and lowering actuator 168. At thefoot of the common base 156 are supporting legs 170 and hemisphericldowel locators 172 which are adapted to fit into the spherical holes 174(FIG. 25) of the main plug 48 for purposes of accurately locating themechanism at every refueling position.

Referring to FIG. 20 the subassembly handling mechanism 178 comprisesdriving means 182 which are associated with the parallel axis gripperend 184. The cable hoist 186 is provided for removal of the small topplug 92 (FIG. 22). Referring again to FIG. 20 there is provided a guidering 188 located on the control rod drive.

A trunnion 190 is provided for mounting on the trunnion support 160 ofthe common support base 156 (FIG. 24). A power level disconnect 192 islocated adjacent the control rod guide, and a guide member 194 isprovided for guiding the small top plug 92 during removal. A rack 196 iscontained in the mechanism for holding a subassembly during therefueling operation.

In order to prevent contamination of the inside of the reactor, a vacuumand gas sweep circuit is provided for purifying the atmosphere adjacentthe subassemblies prior to removal. The inlet portion of the circuitcomprises a valve 198 for a gas supply line which contains an inert gassuch as argon, for example. An outlet 200 leads to a vacuum source forremoval of the gases. Still referring to FIG. 20, a bell housing 202 isprovided for receiving the small top plug 92 during the refuelingoperation. Actuators 204 serve to shift the bell housing 202 between aremote position and a position adjacent the liner tube.

In operation, when it is desired to replace a subassembly, the trunnion190 of the subassembly handling mechanism 178 (FIG. is mounted on thetrunnion support 160 of the common support base 156 (FIG. 24). A crane(not shown) lowers the entire mechanism until the feet 172 of the base156 fit into the selected hemispherical guide holes 174 (FIG. 25)located on the top of the main plug 48. This mechanism is adapted toservice all of the liner tubes regardless of their radial position withrespect to the reactor centers. The support base 156 is so constructedthat the total weight is distributed over a wide expanse of the mainplug so that, even when placed at the maximum angle, the structureisself-supporting and inde pendent of the building crane. Actuators 166and 168 (FIG. 24) position the entire mechanism at the required anglewith respect to the liner tube. When the subassembly handling mechanism178 is so seated on the top of the plug, it is ready for an inclineddownward motion which enables the lower end of the mechanism to engagethe top of the plug sleeve 84 as shown in FIG. 22. The top portion ofthe plug sleeve 84 has a slightly tapered machined outer surface 176which is adapted to be engaged by the subassembly handling mechanism 178when it is placed thereover, an O-ring 180 being provided for sealingpurposes. Thence, the space between the small top plug 92 and the valve198 is evacuated through the outlet 200, and the valve 198 is opened.The cable hoist 186 is adapted topick up the small top plug 92 and liftit into the bell housing 202. Thence, the actuator 204 (FIG. 20) movesthe bell housing to a side position, sliding it along seal plate 208.

After the small top plug has been moved inslde the bell housing 202, thesubassembly handling mechanism is ready to transfer a subassembly, Aparallelogram-type fuel handling mechanism designated generally at 184(FIG. 21) is employed which contains an electrically operatedself-seeking gripper head 210 which contains flexible fingers 212 forgripping the subassemblies. As best seen in FIG. 11, the upper portionof the liner tube 50 is provided with a subassembly gripping ring 214shaped for maintaining correct angular position of the subassembly. Atight contact as at 216 is maintained to permit small angular support ofthe subassembly as at 218. It is noted that all fuel, blanket, andcontrol rods have similar top rings for handling purposes.

A new subassembly, having previously been placed in the rack 196 of thesubassembly handling mechanism, is removed therefrom and lowered intothe liner tube 50 and placed in the decay or storage region 58 of theliner tube 50, as best seen in FIG. 18. Lugs 220 are anchored on theinside wall of the tubular portion 58 of the liner tube 50 forsupporting the subassemblies in the decay or storage region. Thesubassemblies are provided with mating indented or recessed portions 100which are adapted to engage the lugs 220. Thence, a hot subassembly 76is removed from the core (FIG. 1) and is placed in a storage positionadjacent the new subassembly. The

new subassembly is then removed and placed into the core of the reactor,and the decayed subassembly, which has been in storage since the lastrefueling operation, is is removed from the reactor decay region andplaced in the subassembly rack 196 (FIG. 20) of the subassembly handlingmechanism 178.

Then the bell housing 202 is moved back over the plug sleeve and thecable mechanism 186 lowers the small top plug back into position. Thevalve 198 is closed and the space between the valve and the top of thesmall top plug is swept with new argon gas. The subassembly handlingmechanism can then be moved to the canning station (not shown) where ittransfers the decayed subassembly for reprocessing. The cycle isrepeated until all liners requiring refueling have been serviced. It isnoted that normal refueling operations require replacement of only a fewsubassemblies depending upon their residence time and location withinthe reactor. Hence, a relatively few liner tubes will be opened at eachrefueling cycle. The above refueling sequence is illustrated in FIG. 26,which is a view looking downwardly into the core decay region of theline tube 50, wherein the position of the subassembly requiringplacement is designated at 222. The new subassembly 76A is lowered intothe liner tube in the storage or decay region. A hot subassembly 76B isremoved from the core and placed in the storage region adjacent the newone 76A. Then the new subassembly 76A is placed in the core in thevacant position 222. The decayed subassembly 76C which has been instorage, since the last refueling period, is removed from the reactorand placed in the rack 196 of the subassembly handling mechanism 178.

Another refueling cycle which may be employed is illustrated in FIG. 27,which also shows a plan view of the core decay region 58 of the linertube 50. This method of refueling requires a gripper similar to grippermechanism 184 (FIG. 21), except that it requires a double head gripperwhereby two subassemblies may be simultaneously raised or lowered.Referring again to FIG. 27, a new control rod 76D and a new fuelsubassembly 76E are lowered into the storage or decay region 58 of theliner tube 50. The old control rod 76F is moved from the control rodposition 224 to the decay region as shown, and a hot subassembly 76G ismoved from its position 226 in the reactor core to the decay region, asshown. Thence, the control rod 76F and a decayed subassembly 76H whichhas been in storage since the last refueling period are removed from thereactor storage position and brought up into the subassembly handlingmechanism.

Referring next to FIG. 23 there is shown a liner handling mechanismdesignated generally at 228. It will be appreciated that the liner tubesnormally do not require replacement. However, during the life of areactor it is quite possible that damage may occur thereto and hence itsreplacement becomes necessary. The liner handling mechanism 228 isadapted to operate in conjunction with the common support base 156 (FIG.24). The liner handling mechanism 228 is provided with a trunnion 230for mounting on the trunnion support of the common base 156. The linerhandling mechanism 228 further comprises a cable winder 232 associatedwith a liner gripper 234 which grips the under portion of a liner tube50. The side of the handling mechanism is provided with guides 236 whichare power shifted rollers. At the base of the handling mechanism is avalve 238 and a vacuum and gas sweep outlet 240 which operate in amanner similar to that described hereinbefore in connection with thevalve and line 198 and 200 (FIG. 20). After the space between the smalltop plug and the valve 238 has been evacuated, the valve is opened andthe liner gripper 234 is brought down into engagement with the grippergroove 235 (FIG. 22) of the liner tube 50. The liner tube is removed andretained within the handling mechanism and a new liner is inserted intothe reactor. After which, the valve is closed and the space between thevalve and the main plug is swept with new argon gas and the small topplug is replaced. The liner handling mechanism is then moved to aposition remote from the reactor. It is noted that the subassemblyhandling mechanism 178 and the liner handling mechanism 228 are storedin decay pits whereas the common supporting base 156 does not requirestorage in a decay pit.

It will be appreciated that by providing a subassembly decay position 58above the axial blanket in each liner tube 50, substantial handling ofsubassemblies is eliminated. This method of handling the subassembliesis preferable to that employed hereinbefore wherein subassemblies wereremoved from the core and placed in positions around the radialperiphery of the reactor. Such a move required liquid metal coolingwhile the subassembly was being transferred within or being removed fromthe reactor. According to the present invention, the fuel subassembly isallowed to remain in the decay region for a period of time, and henceliquid cooling is not required when the subassembly is removed from thereactor. However, it may be desirable to employ some simple method ofcooling the subassemblies during the out of the reactor handling timebefore they are placed in the canning station. A cooling medium such asgas could be used, if desired.

Although particular embodiments of the invention are herein disclosedfor purposes of explanation, further modifications thereof after studyof this specification, will be apparent to those skilled in the art towhich the invention pertains. Reference should accordingly be had to theappended claims in determining the scope of the invention.

What is claimed and described to be secured by Letters Patent is:

1. A method of replacing a subassembly in a nuclear reactor having areactor vessel, a plurality of liner tubes extending through saidreactor vessel, each of said tubes containing a plurality ofsubassemblies, and a main plug for said reactor vessel, said methodcomprising positioning a subassembly handling mechanism on top of saidrnain plug and aligning said mechanism with a preselected liner tube,removing a small top plug from engagement with the top of said main plugwith a gripper cable mechanism and positioning same within saidsubassembly handling mechanism, lowering a new subassembly into theliner tube, retaining said new subassembly inside the liner tube in adecay region of the reactor, removing a hot subassembly f-rom the coreof the nuclear reactor and placing same in said decay region adjacentthe new subassembly, thence placing the new subassembly in the reactorcore, removing a decayed subassembly which has been retained in thedecay region of the reactor since the last refueling operation,returning the small plug from the subassembly handling mechanism to itsengaged position on top of the main plug, and removing the subassemblyhandling mechanism.

2. A method of replacing a subassembly in a nuclear reactor having areactor vessel, a plurality of liner tubes extending through saidvessel, each of said tubes containing a plurality of subassemblies, anda main plug for said reactor vessel, said method comprising positioninga subassembly handling mechanism on top of said main plug, aligning saidmechanism with respect to a preselected liner tube, evacuating the spacebetween the top of the main plug, and a valve disposed in the handlingmechanism, opening said valve, removing a small top plug from engagementwith the top of said main plug with a gripper cable mechanism andpositioning same within the subassembly handling mechanism, lowering anew subassembly into the liner tube by an electrically operatedselfseeking gripper head of the parallelogram type, retaining said newsubassembly within the liner tube in a decay region of the reactor,removing a hot subassembly from the core of the nuclear reactor andplacing same in said decay region adjacent said new subassembly, thenceplacing the new subassembly in the reactor core, removing a decayedsubassembly which has been retained in the decay region of the reactorsince the last refueling operation, and placing same in a rack disposedwithin the subassembly handling mechanism, lowering the small top plugby the cable mechanism to its closed position, closing the valve of saidsubassembly handling mechanism, and sweeping the space between the valveand the main plug, and removing the subassembly handling mechanism.

3. A method of replacing a subassembly in a nuclear reactor having areactor vessel, a plurality of liner tubes extending through saidvessel, each of said tubes containing a plurality of subassemblies, anda main plug for said reactor vessel, said method comprisinng mountingthe trunnion of a subassembly handling mechanism on the trunnion supportof a support base, positioning said base and said subassembly handlingmechanism on top of said main plug, aligning said handling mechanismwith a preselected liner tube, evacuating the space between the top ofthe main plug and a valve disposed in the handling mechanism, openingsaid valve, removing a small top plug, from its engaged position at thetop of a penetration extending through the main plug, with a grippercable mechanism, and positioning said plug within a bell housing in thesubassembly handling mechanism, moving the bell housing by an actuatorsystem to a remote position, lowering a subassembly into the liner tubeby an electrically operated self-seeking gripper head of theparallelogram type, hooking said new subassembly on the inside wall ofthe liner tube in a decay region of the reactor, removing a hotsubassembly from the core of the nuclear reactor and placing same insaid decay region adjacent the new subassembly, thence placing the newsubassembly in the reactor core, removing a decayed subassembly whichhas been retained in the decay region of the reactor since thelast-refueling operation and placing same in a rack disposed within theouter housing of the subassembly handling mechanism, returning the bellhousing to a position superadjacent its corresponding penetration of themain plug, lowering the small top plug by a cable mechanism to itsclosed position, closing the valve of said subassembly handlingmechanism, sweeping the space between the valve and the top of the smalltop plug, and removing the subassembly handling mechanism.

4. A method of replacing a subassembly in a nuclear reactor having areactor vessel, a plurality of liner tubes extending through saidvessel, each of said tubes containing a plurality of subassemblies and amain plug for said reactor vessel, said method comprising positioning asub- 'assembly handling mechanism adjacent a subassembly cask car andloading one subassembly into the subassembly handling mechanism,mounting the trunnion of said subassembly handling mechanism on thetrunnion support of a support base, positioning said base and saidsubassembly handling mechanism on top of said main plug and fitting thefeet of the base into selected hemispherical guide holes disposed in thetop of the main plug to align said handling mechanism with a preselectedliner tube, moving the handling mechanism downwardly until the mechanismengages the top of a plug sleeve which passes through an inwardlyextending penetration in the main plug, evacuating the space between thetop of the plug sleeve and a valve in the handling mechanism, openingsaid a valve removing a small top plug from engagement with the top ofsaid plug sleeve with a gripper cable mechanism and positioning samewithin a bell housing in the subassembly handling mechanism, moving thebell housing by an actuator system to a remote position with respect tosaid plug sleeve, lowering said new subassembly into the liner tube byan electrically operated self-seeking gripper head of the parallelogramtype, hooking said new subassembly on the inside wall of the liner tube,in a decay region of the reactor, removing a hot subassembly from thecore of the nuclear reactor and placing same in said decay regionadjacent the new subassembly, thence placing the new subassembly in thereactor core, removing a decayed subassembly which has been retained inthe decay region of the reactor since the last refueling operation andplacing same in a rack disposed Within the outer housing of thesubassembly handling mechanism, returning the bell housing to a positionsuperadjacent the plug sleeve, lowering the small top plug by the cablemechanism to its closed position, closing the valve of said subassemblyhandling mechanism, sweeping the space between the valve and the top ofthe small top plug, and removing the subassembly handling mechanism.

References Cited UNITED STATES PATENTS 3,212,982 10/1965 Astley et al17640 FOREIGN PATENTS 877,679 9/1961 Great Britain. 894,228 4/1962 GreatBritain. 897,448 5/ 1962 Great Britain.

10 REUBEN EPSTEIN, Primary Examiner H. E. BEHREND, Assistant Examiner

